3424
Table 1
Tandem Suzuki–dehydrogenation reaction
(1) by trapping the enolate with a triflating reagent, N-phenyltrifluoromethanesulfonimide (Tf2NPh).7
The regiochemistry of enolization of α-amino ketone has been investigated by A. R. Martin et al.8 In
accordance with his result, when the N-substituents are electron-donating groups (methyl, benzyl), only
allylamine-type isomers were detected, and if the substituents are electron-withdrawing groups (Cbz,
Boc), the enamine-type isomers were exclusively observed. As expected, the reaction took place in
such a manner that it gives exclusively allylamine-type triflate (2) rather than enamine-type isomer,
determined by NOE experiments. We have treated the vinyl triflate (2) with boronic acid derivatives
in the presence of a catalytic amount of Pd(PPh3)4 and KOAc in dioxane at 100°C.9 The reaction
mixture provided a compound, which was isolated and characterized to be 3-arylpyrroles. Apparently,
after Suzuki coupling, subsequent dehydrogenation was occurred providing 3-arylpyrroles. In general,
dehydrogenative aromatizations can be achieved by heating (300∼350°C) hydroaromatic compounds
with catalytic amounts of palladium on activated carbon.10 In our reaction, under the Suzuki coupling
condition, the dehydrogenative aromatization11 of the pyrroline (2) took place in tandem mode. This
could be the first example of tandem Suzuki–dehydrogenation reaction to prepare 3-arylpyrroles from
pyrroline.
References
1. Anderson, H. J.; Loader, C. E. Synthesis 1985, 353–364 and references cited therein.
2. Bean, G. T. In Pyrroles. Part 1: The Synthesis of 1H-Pyrroles; Jones, R. A., Ed.; John Wiley and Sons: New York, 1990;
pp. 105–294. Gupta, R. R.; Kumar, M.; Gupta, V. Heterocyclic Chemistry; Springer: Heidelberg, 1999; Vol. 2, pp. 34–43.
3. Babler, J. H.; Spina, K. P. Tetrahedron Lett. 1984, 25, 1659–1660.
4. Gupton, J. T.; Krolikowski, D. A.; Yu, R. H.; Riesinger, S. W.; Sikorski, J. A. J. Org. Chem. 1990, 55, 4735–4740. Gallagher,
P. T.; Palmer, J. L.; Morgan, S. E. J. Chem. Soc., Perkin Trans. 1 1990, 3212–3214.
5. Campi, E. M.; Jackson, W. R.; Nilsson, Y. Tetrahedron Lett. 1991, 32, 1093–1094.
6. Ritter, K. Synthesis 1993, 735–762.
7. Preparation of vinyl triflate (2): To a stirred solution of 1-benzyl-3-pyrrolidinone (1) in anhydrous THF under nitrogen at
−78°C was added sodium bis(trimethylsilyl)amide (1.0 M solution in THF) dropwise. After stirring at −78°C for 1 h, a
solution of N-phenyltrifluoromethanesulfonimide in anhydrous THF was quickly added to the mixture. After agitating at
−78°C for 10 min, the slurry was warmed slowly to 0°C and stirred at 0°C for 3 h. The solvent was removed, and the crude